There is only one thing to do. Just as Felixedet2000 said. If you can't get the client to peacefully return the goods call the police and report a, Theft by Deception, or if the, now X, client refuse to pay for your services; then its a civil matter for the courts. and you seek redress through, Litigation & Civil Procedure .
Actions of any other kind, especially violence, may result in your being, arrested, prosecuted and possibly imprisoned.
Bottom line______________don't take matters into your on hands.